kbuild: Fix <linux/version.h> for empty SUBLEVEL or PATCHLEVEL

expr treats all numbers as decimals, so prepending a zero is safe. Note
that the KERNEL_VERSION() macro still takes three arguments, 3.0 has to be
written as KERNEL_VERSION(3,0,0).

Signed-off-by: Michal Marek <mmarek@suse.cz>